where \(\sim _s\)  denotes the \(S\) -equivalence for semi-stable vector bundles. For an introduction to \(S\) -equivalence, which is weaker than holomorphic equivalence, see section 2 of [1]}. Seshadri proved that (REF ) is an irreducible normal projective variety of dimension \(3g-3\)  [2]}. For \(g \geqslant 3\) , the variety \(\mathcal {M}^B_X\)  has rational singularities, see [3]} Remark 2.7, and the singular locus, which has codimension greater than one, corresponds to (\(S\) -equivalence classes of) semi-stable vector bundles which are not stable, see theorem 1 of [1]}.
